Is Blue Algae Toxic to Humans? Understanding the Risks of Cyanobacteria

November 25, 2025 •

4 min read

According to the Centers for Disease Control and Prevention, some types of blue-green algae, more accurately known as cyanobacteria, can produce toxins harmful to humans and animals. This makes it crucial for anyone enjoying freshwater bodies to understand the potential risks associated with these naturally occurring organisms, especially during algal blooms.

Quick Summary

Blue-green algae, or cyanobacteria, can produce harmful toxins during blooms. Exposure can cause health issues ranging from skin irritation to liver damage. Identification is visual, but toxicity requires lab testing. Avoid contact with affected water and never boil it, as this can release more toxins.

Key Points

Cyanobacteria, Not Algae: Blue-green algae is actually a type of bacteria that can produce harmful toxins, not a plant-like organism.
Toxicity is Unpredictable: You cannot tell if a bloom is toxic just by its appearance; laboratory testing is required to confirm the presence of cyanotoxins.
Health Risks Vary: Exposure through swallowing, skin contact, or inhalation can cause symptoms ranging from gastrointestinal upset and skin irritation to more severe liver or nervous system damage.
Do Not Boil Contaminated Water: Boiling water with cyanobacteria is dangerous as it can rupture the bacteria cells and release more toxins into the water.
When in Doubt, Stay Out: The safest action is to avoid contact with any water that shows signs of a potential algal bloom, including keeping children and pets away.
Prevention Through Nutrient Reduction: Managing and reducing nutrient runoff from fertilizers and septic systems is a key strategy for minimizing the frequency and severity of blooms.

What Exactly is Blue Algae?

Despite its common name, blue-green algae is not actually algae but a type of bacteria known as cyanobacteria. These microscopic, photosynthetic organisms have existed for billions of years and are a natural part of most aquatic ecosystems, including lakes, rivers, and ponds. Under favorable environmental conditions, such as warm water, high nutrient levels (from runoff), and plenty of sunlight, cyanobacteria can multiply rapidly to form what is known as an algal bloom.

The Dangers of Cyanotoxins

During these blooms, some species of cyanobacteria produce potent natural poisons called cyanotoxins. It is impossible to tell if a bloom is toxic just by looking at it, as some species produce toxins while others do not. The risk to human health comes from contact with, inhalation of, or ingestion of these toxins, which can affect different parts of the body.

Common health effects from exposure include:

Gastrointestinal issues: Swallowing contaminated water can lead to nausea, vomiting, diarrhea, and stomach pain.
Skin and eye irritation: Direct contact with a bloom can cause rashes, hives, and itchy eyes.
Respiratory problems: Inhaling airborne water droplets from a contaminated source may lead to a cough, wheezing, or a sore throat.
More severe symptoms: Ingestion of larger quantities of toxins can potentially cause liver damage (hepatotoxins) or damage to the nervous system (neurotoxins), though these are less common in humans.

How to Identify a Harmful Algal Bloom

While a lab test is the only definitive way to determine toxicity, there are visible signs that indicate the presence of a bloom, prompting caution. Harmful blooms can vary in appearance, but common indicators include:

Appearance: The water may look like pea-soup, spilled paint, or be covered with a thick, scummy layer. Colors range from blue-green to green, brownish-green, red, or white.
Smell: Decaying blooms can emit unpleasant odors, often described as septic or musty.
Location: Blooms are often blown to shorelines by wind and can form mats on the surface or along the water's edge.
Stick Test: If you can pick up a clump with a stick, it is more likely a harmless filamentous green algae. Cyanobacteria usually break apart easily.

Cyanobacteria vs. Harmless Algae: A Comparison


Feature	Cyanobacteria (Blue-Green Algae)	Harmless Green Algae
Classification	Bacteria (Prokaryote)	Algae (Eukaryote)
Appearance (Bloom)	Often looks like spilled paint or thick scum; multiple colors (blue-green, brown, red).	Clumps together in strands or filaments; often bright green and looks like wet hair or moss.
Smell (Decaying)	Strong, often unpleasant musty or septic odors.	Generally less pronounced odor, often more 'grassy'.
Toxin Production	Some species produce harmful cyanotoxins.	Typically non-toxic, though can be a nuisance.
Buoyancy	Can regulate buoyancy, moving up and down in the water column.	Generally sinks or remains stationary if undisturbed.
Identification	Cannot confirm toxicity by sight; requires lab testing.	Can often be identified visually, but still best to be cautious with any bloom.

Protective Measures and Prevention

Protecting against exposure requires a proactive and cautious approach. When in doubt, it is best to assume a bloom is harmful.

Immediate precautions when a bloom is suspected include:

Stay out of the water. Avoid swimming, wading, boating, or any activity that causes water to splash on you or others.
Keep pets and livestock away from the water. Dogs are particularly susceptible to poisoning as they may ingest algae by drinking the water or licking it off their fur.
Do not drink untreated surface water, and do not use it for cooking or preparing food.
Do not boil the water. This can break open the cyanobacteria cells, releasing higher concentrations of toxins into the water.
Rinse off with fresh, clean water immediately if contact occurs.
Do not eat fish organs from water with a history of blooms, as toxins can accumulate in them.

Long-term prevention of harmful algal blooms involves reducing the nutrient load in water bodies:

Use phosphate-free detergents and cleaning products.
Reduce or eliminate fertilizer use on lawns, especially near shorelines.
Ensure septic systems are properly maintained and not leaking.
Maintain naturalized shorelines with vegetation to reduce runoff.

Conclusion

While not all blue-green algae is toxic, the presence of a harmful algal bloom, or cyanobacteria bloom, poses a clear health risk to humans and animals. Since visual inspection alone cannot determine if toxins are present, the safest approach is to treat all blooms as potentially hazardous. By understanding the signs of a bloom and following critical safety precautions—including staying out of the water, keeping pets away, and avoiding untreated water—you can effectively protect yourself and your loved ones. Public health advisories and sustained environmental management to reduce nutrient runoff are key tools in preventing these blooms and ensuring the safety of our recreational waterways for everyone. For more information, consult the CDC's guidance on harmful algal blooms.

Frequently Asked Questions

Common signs of a HAB include discolored water that may appear like pea soup or spilled paint, thick scum or foam on the surface, and an unpleasant, musty, or septic-like odor.

If you have come into contact with affected water, wash yourself thoroughly with clean water immediately. If you experience symptoms such as nausea, vomiting, or skin irritation, seek medical attention and inform your doctor about the potential exposure.

No, you should never boil water containing blue-green algae. Boiling does not destroy the toxins and can actually cause more of them to be released into the water.

Yes, pets are highly susceptible to blue-green algae toxins, especially dogs. They are at risk from drinking contaminated water or from licking algae off their fur after swimming. Symptoms can include vomiting, diarrhea, and seizures.

It is possible for cyanobacteria to affect a tap water supply drawn from a nearby reservoir, although public water systems are required to monitor and treat for toxins. If a public advisory is issued, follow official guidance and use an alternative water source.

Eating fish caught in affected water is generally considered low-risk if only the fillets are consumed, as toxins concentrate in organs like the liver. However, as a precaution, some advisories recommend waiting a week or two after a bloom subsides before fishing.

The primary drivers of blooms are high water temperatures and excess nutrients, particularly nitrogen and phosphorus, which often come from agricultural and stormwater runoff and faulty septic systems.